The present invention relates to a method and a device for stacking flat mailings, the mailings including differences with respect to their format (length, height), their thickness, and their stiffness. Flexible and stiff mailings in large and small formats (e.g.: DIN B4 and small postcards) should be capable of being processed both separately and in a mixture. The mailings of different length, height, thickness and stiffness are transported as individual mailings with a minimum spacing in covering belts and, for the purpose of further processing or for storage, should be aligned in the stack exactly on two edges (front and lower edge).
A method and a device for stacking flat mailings is disclosed in DE 27 14 520 A1, wherein an underfloor belt and a stack support detachably connected to the latter being moved away from the stacking point as the stack grows, and the mailings transported to a stacking point by a covering belt system being moved to a stop by means of stacking belts and a stacking roll, forming a stack.
A solution was known in which a mechanical limit switch is located directly in front of the stacking roll underneath the stacking belt, projecting geometrically into the stream of letters, and is thus pressed when a mailing moves past it. This limit switch outputs the signal to a driven underfloor belt to move the underfloor belt as long as the switch is pressed. Since the switch is arranged underneath the belt, it is able to detect only the lower region of a mailing. Depending on the skewed position of a mailing, the switch is not touched at all and thus does not output any movement signal to the underfloor belt and, in the other case, the mailing presses permanently on the switch, the underfloor belt moves, together with the mailing, away from the stacking roll, until the switch has reached its initial position again. This type of switch reacts only to a defined stacking force. It is not able to react to the individually detected specifically different mailings. Thus, for all the different types of mailing, there is only a single stacking force within the stack, which is not optimal for all types of mailing.
JP 08 259 080 AA discloses the fact that, during stacking, an exact alignment can be achieved if the stack is moved away from the stacking point on the basis of the thickness of the objects, and the speed of this movement is corrected on the basis of stacking forces measured at a plurality of points at the stacking point. JP 08 113 410 AA also teaches controlling this movement on the basis of the thickness and of the stacking force. The goods to be stacked in this case exhibit great differences with regard to their size, thickness and condition.
Furthermore, devices for intermediately stacking mailings were known in which the control for moving a stack support is carried out on the basis of a force measurement (DE 1 235 818 A, DE 195 47 292 A) or on the basis of a force and thickness measurement in the case of mailings with relatively small size and thickness differences (U.S. Pat. No. 3,918,704 A).